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Acid-base reactions and oxidation-reduction reactions are two common and very different classes of chemical reactions. While acid-base reactions involve ionic exchanges between an acidic species and a basic species without any change in the oxidation states of any atom involved, oxidation-reduction reactions involve an exchange of electrons that does affect the oxidation states. oxidation.
In addition to this fundamental difference, acid-base and oxidation-reduction reactions also involve different types of chemical reagents. The acid-base ones require the intervention of acids and bases, while the oxidation-reduction ones require oxidizing agents and reducing agents. There are some chemical compounds that can participate in both kinds of chemical reactions , while another group of compounds can only participate in one of the two. Non-oxidizing acids are an example of chemicals that belong to this second group.
To understand what it means for a substance to be a non-oxidizing acid, we must first understand what is an acid and what is an oxidant.
What is an acid?
There are several different concepts of acids and bases. From the point of view of the Arrhenius acid-base theory, an acid is a substance capable of ionizing in aqueous solution, releasing H + ions (protons) into the solution. From the point of view of the Brønsted and Lowry theory, acids are those substances capable of donating a proton to a base; while Lewis theory defines acids as electron-deficient chemical species capable of receiving a pair of electrons in a dative covalent bond.
Of all the definitions, Lewis’s is the broadest and is the one that encompasses all the substances we know as acids. The concept implies that for a substance to act as an acid it must react with another that acts as a base.
What is an oxidizing agent?
Oxidizing agents are substances capable of removing one or more electrons from another chemical substance. During the process, the oxidizing agent is reduced, while the other substance (called the reducing agent) is oxidized. In other words, oxidizing agents are substances capable of oxidizing other substances, and it is from this capacity that they get their name.
The main characteristic of a good oxidizing agent is that it has a high reduction potential. This indicates that they have a strong tendency to reduce, which in turn implies that they have a large capacity to oxidize other species.
What is a non-oxidizing acid?
Based on the previous concepts, we can establish a general definition of what a non-oxidizing acid is . In this sense, we can say that a non-oxidizing acid is any chemical substance capable of donating a proton to another or capable of accepting a pair of electrons in the form of a dative covalent bond, but which does not have the tendency to reduce or oxidize to other chemical species . In other words, a non-oxidizing acid is an acid with a relatively low reduction potential.
Care should be taken with this definition, as it can be misleading. All Arrhenius or Brønsted and Lowry acids produce protons or H + ionsand these can be reduced to molecular hydrogen, thus acting as oxidizing agents. From this point of view, we could then say that all Arrhenius or Brønsted and Lowry acids are oxidizing acids. However, this is not considered so. To avoid confusion, oxidizing acids are considered to be those that, in solution, produce some species whose reduction potential is greater than that of hydrogen. Since reduction potentials are measured in terms of the standard hydrogen electrode, which is assigned a value of zero by definition, the concept of an oxidizing agent is reinterpreted as a substance that has a positive reduction potential.
We can define, then, a non-oxidizing acid as any chemical substance capable of donating a proton to another or capable of accepting a pair of electrons in the form of a dative covalent bond, and that does not generate any species with a positive or negative reduction potential. greater than that of hydrogen .
Characteristics of non-oxidizing acids
Non-oxidizing acids have the general characteristics of acids and some others that make them non-oxidizing. These features are:
- They are sour-tasting substances.
- They are generally soluble in water.
- They produce aqueous solutions with acidic pH (less than 7).
- They can be both strong and weak acids.
- They have no tendency to pick up electrons or shrink.
- They do not generate ions with a reduction potential greater than or equal to zero.
- They are not easily reduced.
- They can be corrosive and irritating to the skin and other organic tissues.
- They can react with active metals to produce gaseous hydrogen.
Why are some acids oxidizing and others not?
The reason there are oxidizing acids and non-oxidizing acids is simply that acidity and reducing potential are not necessarily related to each other. Acidity is a function of the stability of an acid relative to the stability of its conjugate base. Substances that are highly acidic either have highly unstable structures that stabilize upon conversion to the conjugate base, or form conjugate bases that are particularly stable; or both can also occur at the same time.
Instead, the ability of a substance to behave as an oxidizing agent depends on the reduction potential and the stability of the reduced species, which is different from the conjugate base.
Two illustrative examples of the difference between oxidizing and non-oxidizing acids
Many mineral acids are strong acids and at the same time good oxidizing agents. A typical example is nitric acid (HNO 3 ), which, when dissociated in water, produces nitrate ions (NO 3 – ) that can be easily reduced to nitrogen monoxide (NO). Nitrate is a good oxidizer, as it has a positive reduction potential of +1.10 V, which is relatively high.
The reason that nitrate is a good oxidizing agent is because, in all stable resonance structures for this ion, the central nitrogen atom carries a positive formal charge, despite the fact that nitrogen is a highly electronegative element. This makes nitrogen have a strong tendency to pick up electrons and reduce.
This is in contrast to hydrochloric acid (HCl), which is also a strong mineral acid but is not an oxidizing agent. The conjugate base of hydrochloric acid, that is, the chloride ion (Cl – ) has its full octet and also has a negative charge on a highly electronegative atom, which is the ideal situation. In fact, chlorine can’t be reduced beyond the chloride ion, so it’s impossible for HCl to act as an oxidizing agent (unless H+ is the oxidant, but as we saw earlier, H+ doesn’t count).
Examples of non-oxidizing acids
There are many non-oxidizing acids. Most organic acids are non-oxidizing, while all hydrocids are also non-oxidizing. Below is a list of 13 non-oxidizing acids commonly used in the laboratory.
| Hydrofluoric acid (HF) | Hydrogen sulfide ( H2S ) | Benzoic Acid (C 6 H 5 COOH) |
| Hydrochloric Acid (HCl) | Carbonic acid (H 2 CO 3 ) | Chloroacetic acid (ClCH 2 COOH) |
| Hydrobromic Acid (HBr) | Acetic acid (CH 3 COOH) | Formic Acid (HCOOH) |
| Hydroiodic Acid (HI) | Phosphoric acid (H 3 PO 4 ) | Trifluoroacetic acid (CF 3 COOH) |
| Sulfuric Acid (H 2 SO 4 ) |
References
CAMEO Chemicals. (n.d.). Acids, Strong Non-oxidizing . CAMEO. https://cameochemicals.noaa.gov/react/1
Chang, R. (2008). Physical Chemistry for the Chemical and Biological Sciences (3rd ed.). MCGRAW HILL EDUCATION.
M Olmo, RN (nd). Oxidation-Reduction Potentials . HyperPhysics. http://hyperphysics.phy-astr.gsu.edu/hbasees/Chemical/redoxp.html
New Jersey Institute of Technology. (n.d.). Acids (Non-Oxidizing) . NJIT.EDU. https://www.njit.edu/environmentalsafety/sites/njit.edu.environmentalsafety/files/Acids_%28Non-Oxidizing%292-fillable-logoFix_0.pdf
Acid-base reaction | Clear definition of . (2016, May 11). i-sciences.com. https://www.i-ciencias.com/pregunta/46889/definicion-clara-de-un-quotacido-no-oxidantequot
Meanings (2015, January 30). Oxidant meaning . Meanings. https://www.meanings.com/oxidant/
Arrhenius Theory of Acids and Bases . (2015, May 21). Chemicals.Net. https://www.quimicas.net/2015/05/arrhenius-theory-of-acids-y-bases.html
